One special feature offered in mobile communications systems is group communication. Conventionally group communication has been available only in trunked mobile communications systems, such as Professional Radio or Private Mobile Radio (PMR) systems, such as TETRA (Terrestrial Trunked Radio), which are special radio systems primarily intended for professional and governmental users. Group communication is becoming available also in public mobile communications systems. New packet-based group voice and data services are being developed for cellular networks, especially in GSM/GPRS/UMTS network evolution, wherein the approach is based on the idea of a group communication service being provided as a packet-based user or application level service so that the underlying cellular network only provides the basic connections (i.e. IP connections) between the group communications applications in user terminals and the group communication service. When this approach is employed in push-to-talk communication, the concept is also referred to as a push-to-talk over cellular (PoC) network.
Generally, in group voice communication with a “push-to-talk, release-to-listen” feature, a group call is based on the use of a pressel (PTT, push-to-talk switch) in a telephone as a switch: by pressing a PTT the user indicates his or her desire to speak, and the user equipment sends a service request to the network. The network either rejects the request or allocates the requested resources on the basis of predetermined criteria, such as the availability of resources, priority of the requesting user, etc. At the same time, a connection is established also to all other active users in the specific subscriber group. After the voice connection has been established, the requesting user may talk and the other users may listen on the channel. When the user releases the PTT, the user equipment signals a release message to the network, and the resources are released. Thus, the resources are reserved only for the actual speech transaction or speech item, instead of reserving the resources for a “call”.
PMR systems are widely employed by authorities and specific user groups. However, it is not possible to communicate with a PMR group without an appropriate PMR capable terminal. Such communications need could arise for instance when additional volunteers help officials to locate a missing person. A patent application publication US 2002/0196781 discloses a method of routing calls to a TETRA system. There is a gateway between the TETRA network (connected to the TETRA core network exchange element) and an IP network. A terminal in the IP network (for instance in a mobile network connected to the IP network) may communicate with the TETRA system by emulating a TETRA terminal, whereby the TETRA protocol data is transferred between the TETRA network and the terminal over the IP. Thus, the terminals outside the TETRA network need to be capable of at least encoding and decoding data according to the TETRA protocol.